@Sweetridge If the plan you had previously was grandfathered, then you wouldn't be able to schedule a change back to that plan as it's no longer offered. Otherwise, just schedule the (Change at Next Renewal) option now as suggested.
How did you change your plan? Did you change it Now or you schedule change on next renewal? If you changed NOW then you have been charged full amount. Otherwise you can modify (I think) your plan change.
I doubt agent will reverse your decision of plan change but you can always try to talk to agent and explain your mistake.
@Sweetridge was the plan already changed today or did you just schedule a future plan change on renewal?
If a future plan change, you can go back and cancel it
But if the plan was already changed amd PM took money for the new plan, the I am afraid you cannot do much. PM is a prepaid provider. And before you forget, make a schedule plan change in next renewal to change back to your old plan